Precisely. It’s just that they kill thousands of people with each attack.
Thagoids may not think of individuals in the same way we do. If they do follow a biological pattern similar to Ants or Bees, then to then they may look at the collective hole. It might be like punching someone in the face. You might give them a bloody nose or a black eye, but are extremely unlikely to kill them. And here that equates to killing a few thousand individual units, which in the big picture does nothing much to the whole of billions other than making a point.
And indeed they may think about themselves in the same way. Killing a few hundred Thargoids and ships (assuming they are separate entities) is attacking but not destroying the whole. They may not have any concept of mourning, or loss, beyond noting that resources have been lost.
Taking this further they may not see what is happening as a small war, or anything like it. It has already been indicated that they might not think about territory in the same way we do. They may not have territory at all. Just places they go for resources. Something has happened to one of there resource sites (the Pleiades) and they are reacting to it. Taking steps to stop that interference.
Thargoids may be a very reactionary species. They may be quite poor at things like being pro-active. They certainly may have plans, but they may follow very pre-defined patterns that have been shown to be successful. When something happens that interrupts that planned activity they react to it. They may not have anything like emotions either. They may not feel anger or sadness, joy or lust. Or have concepts of good or evil. They may simply respond to a threat with self defence, or where they thing something is a threat to their resources, will attack in pre-emptively, which make be as close to pro-active as they can get.
Remember DB has said a few times he is fascinated in looking at what would happen if we met a truly alien species. One that does not conform with our ways of thinking at all. I don't think Thargoids are like Klingons, or Shadows, or Kilrathi, or Drak. I think they will be closer to Ridley Scott and Hans Ruedi Giger's Aliens, but with high technology and far more intelligence. Not evil, just doing their thing, it might just be that you become part of their thing.